It is well known in the art that sheets or webs of expanded polystyrene, with opposed surfaces or "skins" of substantially greater density than the intermediate core have many desirable properties for many end product applications. For example, U.S. Pat. No. 3,468,467 to S. W. Amberg, which is now assigned to assignee of this application, shows a nestable, two-piece disposable drinking cup for hot beverages in which the sidewall element is fabricated from a blank which is cut from a web of expanded polystyrene having dense surface skins, and co-pending application Ser. No. 159,443 of S. W. Amberg et al., which is assigned to the assignee of this application now U.S. Pat. No. 3,760,968, shows a composite container comprised of an inner glass envelope and, attached to and surrounding the exterior thereof, a protective and externally decorated sleeve fabricated from a web of oriented polystyrene.
As is shown, for example, in U.S. Pat. No. 3,560,600 to V. L. Gliniecki, webs of expanded polystyrene having dense surface skins may be produced from warm, blowing agent-containing, tubular extrudates of such material by quickly chilling the inner and outer surfaces of the extrudate as it emerges from the die to inhibit further expansion of the residual blowing agent within the extrudate in the surface portions thereof, by allowing the residual blowing agent in the inner or core portion of the extrudate to continue to expand for some finite period of time, by drawing the extrudate over an internally cooled mandrel of greater diameter than the extrusion die to size the diameter of the tube and to cool substantially all remaining residual blowing agent, and by slitting the tube as it passes from the die along one or more longitudinally extending lines to form one or more webs therefrom. Such an arrangement will work reasonably well when the extrudate is comprised of a substantial portion of a rubber-modified, impact grade of polystyrene and/or when the end products to be formed from the webs of such polystyrene do not require that the mandrel or radially innermost surface of the extrudate be of high quality, for example, the quality required to permit attractive printing or decorating material to be placed thereon.
The tubular extrusion arrangement heretofore described can, however, lead to surface quality defects on the web surface which comprised the radially innermost or mandrel-contacting side of the extrudate from such web was formed. These defects can arise by the "plating out" or accumulation of material from the extrudate onto the mandrel by virtue of the tight, surface to surface contact between the extrudate and the mandrel which is required for proper cooling of the extrudate. Such accumulation of material, which has been found to be especially severe in the case of extrudates comprised of a substantial portion of general purpose grades of polystrene because of the inherently brittle character of such grades, causes scratching or marring of the surface of the extrudate which passes thereover, and this is particularly objectionable when it is desired to subsequently apply printing or decorative material to such surface. To be able to operate at all according to such an arrangement with general purpose grades of polystyrene, it is necessary to very frequently shut the extrusion line down to permit the mandrel to be cleaned or sand blasted, and the costliness of such shutdowns substantially detracts from the economic advantages otherwise to be gained from the substitution of less expensive general purpose grades of polystyrene in applications where impact grades previously were used.